1. Field of the Invention:
The present invention relates to a method for producing an .alpha.-aspartylphenylalanine derivative (hereafter simply referred to as an .alpha.-AP derivative) represented by formula (2), from a .beta.-aspartylphenylalanine derivative (hereafter simply referred to as a .beta.-AP derivative): ##STR2## wherein R.sub.1 and R.sub.2 represent hydrogen or an alkyl group having 1 to 4 carbon atoms.
2. Description of the Related Art:
For production of .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereafter simply referred to as .alpha.-APM) useful as a novel sweetener, a variety of methods are known.
Among them, most of the methods comprise protecting the amino group of L-aspartic acid with a protecting group such as, for example, a carboxybenzoxy group, a formyl group, a hydrogen halide, etc., converting into the corresponding anhydride, condensing the anhydride with L-phenylalanine methyl ester to form N-protected L-aspartyl-L-phenylalanine methyl ester, then splitting the protecting group off to obtain .alpha.-APM. However, according to these methods, .beta.-L-aspartyl-L-phenylalanine methyl ester (hereafter simply referred to as .beta.-APM) is an unavoidable by-product.
On the other hand, a method is known (Published Examined Japanese Patent Application No. 277696/1986) for producing .alpha.-AP derivatives (formula (2)) and .alpha.-aspartylphenylalanine anhydride derivatives (hereafter simply referred to as DKP derivatives) from .beta.-AP derivatives (formula (1)).
However, the yield of the desired .alpha.-AP derivatives is low and large quantities of by-products are produced. It is also disadvantageous in that large quantities of DKP derivatives are formed.
In the case of converting the DKP derivatives to the .alpha.-AP derivatives, its yield is poor and large quantities of by-products are also produced. For this reason, it is desired that the DKP derivatives not be formed. In order to solve this problem, it was extremely important to develop a technique for preventing the formation of DKP derivatives and to improve the yield of .alpha.-AP derivatives. That is, when the .alpha.-AP derivatives can be prepared from the .beta.-AP derivatives in a high yield, such greatly contributes industrially since the .alpha.-AP derivatives can readily be converted to .alpha.-APM.